From e8a63124746761e743d69ea510194de2ada4034f Mon Sep 17 00:00:00 2001
From: Manuel Drehwald <git@manuel.drehwald.info>
Date: Mon, 22 Dec 2025 11:18:35 -0800
Subject: [PATCH] add new configure_cmake option to let projects set cc/cxx
 flags

---
 src/bootstrap/src/core/build_steps/llvm.rs | 60 ++++++++++++++++------
 1 file changed, 44 insertions(+), 16 deletions(-)

diff --git a/src/bootstrap/src/core/build_steps/llvm.rs b/src/bootstrap/src/core/build_steps/llvm.rs
index 51a791daef28..8bf8f6f56c73 100644
--- a/src/bootstrap/src/core/build_steps/llvm.rs
+++ b/src/bootstrap/src/core/build_steps/llvm.rs
@@ -63,6 +63,25 @@ impl LlvmBuildStatus {
     }
 }
 
+/// Allows each step to add C/Cxx flags which are only used for a specific cmake invocation.
+#[derive(Debug, Clone, Default)]
+struct CcFlags {
+    /// Additional values for CMAKE_CC_FLAGS, to be added before all other values.
+    cflags: OsString,
+    /// Additional values for CMAKE_CXX_FLAGS, to be added before all other values.
+    cxxflags: OsString,
+}
+
+impl CcFlags {
+    fn push_all(&mut self, s: impl AsRef<OsStr>) {
+        let s = s.as_ref();
+        self.cflags.push(" ");
+        self.cflags.push(s);
+        self.cxxflags.push(" ");
+        self.cxxflags.push(s);
+    }
+}
+
 /// Linker flags to pass to LLVM's CMake invocation.
 #[derive(Debug, Clone, Default)]
 struct LdFlags {
@@ -527,7 +546,7 @@ impl Step for Llvm {
             cfg.define("LLVM_VERSION_SUFFIX", suffix);
         }
 
-        configure_cmake(builder, target, &mut cfg, true, ldflags, &[]);
+        configure_cmake(builder, target, &mut cfg, true, ldflags, CcFlags::default(), &[]);
         configure_llvm(builder, target, &mut cfg);
 
         for (key, val) in &builder.config.llvm_build_config {
@@ -633,6 +652,7 @@ fn configure_cmake(
     cfg: &mut cmake::Config,
     use_compiler_launcher: bool,
     mut ldflags: LdFlags,
+    ccflags: CcFlags,
     suppressed_compiler_flag_prefixes: &[&str],
 ) {
     // Do not print installation messages for up-to-date files.
@@ -761,23 +781,21 @@ fn configure_cmake(
         .define("CMAKE_ASM_COMPILER", sanitize_cc(&cc));
 
     cfg.build_arg("-j").build_arg(builder.jobs().to_string());
+    let mut cflags = ccflags.cflags.clone();
     // FIXME(madsmtm): Allow `cmake-rs` to select flags by itself by passing
     // our flags via `.cflag`/`.cxxflag` instead.
     //
     // Needs `suppressed_compiler_flag_prefixes` to be gone, and hence
     // https://github.com/llvm/llvm-project/issues/88780 to be fixed.
-    let mut cflags: OsString = builder
+    for flag in builder
         .cc_handled_clags(target, CLang::C)
         .into_iter()
         .chain(builder.cc_unhandled_cflags(target, GitRepo::Llvm, CLang::C))
-        .filter(|flag| {
-            !suppressed_compiler_flag_prefixes
-                .iter()
-                .any(|suppressed_prefix| flag.starts_with(suppressed_prefix))
-        })
-        .collect::<Vec<String>>()
-        .join(" ")
-        .into();
+        .filter(|flag| !suppressed_compiler_flag_prefixes.iter().any(|p| flag.starts_with(p)))
+    {
+        cflags.push(" ");
+        cflags.push(flag);
+    }
     if let Some(ref s) = builder.config.llvm_cflags {
         cflags.push(" ");
         cflags.push(s);
@@ -789,7 +807,8 @@ fn configure_cmake(
         cflags.push(format!(" --target={target}"));
     }
     cfg.define("CMAKE_C_FLAGS", cflags);
-    let mut cxxflags: OsString = builder
+    let mut cxxflags = ccflags.cxxflags.clone();
+    for flag in builder
         .cc_handled_clags(target, CLang::Cxx)
         .into_iter()
         .chain(builder.cc_unhandled_cflags(target, GitRepo::Llvm, CLang::Cxx))
@@ -798,9 +817,10 @@ fn configure_cmake(
                 .iter()
                 .any(|suppressed_prefix| flag.starts_with(suppressed_prefix))
         })
-        .collect::<Vec<String>>()
-        .join(" ")
-        .into();
+    {
+        cxxflags.push(" ");
+        cxxflags.push(flag);
+    }
     if let Some(ref s) = builder.config.llvm_cxxflags {
         cxxflags.push(" ");
         cxxflags.push(s);
@@ -811,6 +831,7 @@ fn configure_cmake(
     if builder.config.llvm_clang_cl.is_some() {
         cxxflags.push(format!(" --target={target}"));
     }
+
     cfg.define("CMAKE_CXX_FLAGS", cxxflags);
     if let Some(ar) = builder.ar(target)
         && ar.is_absolute()
@@ -970,7 +991,13 @@ impl Step for Enzyme {
 
         builder.config.update_submodule("src/tools/enzyme");
         let mut cfg = cmake::Config::new(builder.src.join("src/tools/enzyme/enzyme/"));
-        configure_cmake(builder, target, &mut cfg, true, LdFlags::default(), &[]);
+
+        let mut cflags = CcFlags::default();
+        // Enzyme devs maintain upstream compability, but only fix deprecations when they are about
+        // to turn into a hard error. As such, Enzyme generates various warnings which could make it
+        // hard to spot more relevant issues.
+        cflags.push_all("-Wno-deprecated");
+        configure_cmake(builder, target, &mut cfg, true, LdFlags::default(), cflags, &[]);
 
         // Re-use the same flags as llvm to control the level of debug information
         // generated by Enzyme.
@@ -1090,7 +1117,7 @@ impl Step for Lld {
             ldflags.push_all("-Wl,-rpath,'$ORIGIN/../../../'");
         }
 
-        configure_cmake(builder, target, &mut cfg, true, ldflags, &[]);
+        configure_cmake(builder, target, &mut cfg, true, ldflags, CcFlags::default(), &[]);
         configure_llvm(builder, target, &mut cfg);
 
         // Re-use the same flags as llvm to control the level of debug information
@@ -1213,6 +1240,7 @@ impl Step for Sanitizers {
             &mut cfg,
             use_compiler_launcher,
             LdFlags::default(),
+            CcFlags::default(),
             suppressed_compiler_flag_prefixes,
         );